Descending projections from auditory cortex have been postulated to play a role in a wide variety of functions including selective attention, sound localization, gain control, and frequency selectivity. One of the largest of the descending projections is to the inferior colliculus. We intend to use a combination of anatomical tracing and electron microscopic techniques to elucidate the targets of these projections. Cortical projections terminate in the same area of the inferior colliculus as collicular cells that have ascending projections and cells that have descending projections. Therefore our first two goals are to identify if these cortical projections contact cells in the inferior colliculus that have ascending projections to the medial geniculate, or cells that have descending projections to the superior olivary complex and cochlear nucleus. To better understand the functional role of the pathways, our third goal is to determine whether cells in the inferior colliculus that contain y-aminobutyric acid (GABA), an important inhibitory neurotransmitter, are also contacted by cortical projections. Answers to these questions are necessary to ascertain which brainstem circuits are modulated by cortical projections and what role they may play in both normal and diseased auditory systems.